Heme is required for structure and function of cytochromes, catalase, sulfite reductase, and other hemoproteins. In anaerobic yeast respiratory cytochromes are not made, and normal mitochondria are absent. Primitive promitocondria are observed which develop into normal organelles when air is admitted. In the absence of heme (either owing to anaerobiosis or mutation) synthesis of ergosterol and oleic acid cannot occur, and normal membranes cannot develop, unless ergosterol and Tween 80 (a source of oleic acid) are added to the medium. The development of mitochondrial enzymes, and sterol containing plasma membranes can therefore be studied in heme and sterol mutants. Another feature of membrane structure is the presence of glycosylated proteins. Dolichol phosphate is an important cofactor in the synthesis and attachment of oligosaccharide side chains to membrane proteins. These long chain 2,3-dihydropolyprenols are synthesized from farnesyl pyrophosphate and thus form a branch point in sterol biosynthesis. For this reason, isolated sterol mutants in yeast are limited to those blocked after squalene. New strategies therefore need to be found to obtain early sterol mutants and/or dolichol phosphate mutants. Such yeast strains would be very helpful for a study of the relationship between these two pathways, and for an investigation of the assembly of membranes. A contribution to understanding the synthesis and function of glycoproteins will be significant in elucidating their postulated role in contact inhibition and other inter-cellular recognition phenomena. The lessening of contact inhibition in malignant cells has been attributed to alterations in glycoprotein structure. Membranes of transformed cells show considerably lower concentrations of certain glycoproteins than membranes of normal cells.